Antipyrine is a "marker" for a family of drugs metabolized by hepatic enzymes. Aging subjects show a slower degradation rate of the drug than younger subjects. This age effect is however mainly secondary to the fact that older subjects smoke fewer cigarettes and drink less caffeine (coffee and tea) than do younger subjects. Both of these habits could be shown by multivariate analysis to account for more of the variance in drug metabolism than aging per se. Thus in the evaluation of aging effects in man, differences could be erroneously attributed to "biological aging processes" which are in fact due to differences in habits or in behavior patterns of different age groups.